Why didn't Mina Harker realize she was under Dracula's spell when she witnessed her friend fall prey to him, too? Wasn't it obvious?

Bram Stoker's Dracula truly shaped the course of fiction and film. (Would you believe that Dracula has appeared in more films than any fictional character except maybe Sherlock Holmes?) But actually, most readers find the last half of the original novel to be long and tedious. Stoker wrote Dracula at the end of the Victorian period when the reading public expected novels to last a long time. But to some, this intentional drawing out of the plot also introduced flaws, such as the one you suggest.

Mina Harker, in her journal, details many of the things known about vampires:

They do not die.

They can be as strong as twenty men.

They can direct the elements (storms, fog, etc.).

They can command rats, owls, bats, and wolves.

They can grow large or small at will.

They can vanish.

They can appear in different forms.

Some later renditions of vampire stories, including Dracula, also suggest that vampires have the power to somehow "hypnotize" their desired victims, casting a spell so that the victim — despite knowing otherwise — gives him- or herself to the vampire almost willingly. Perhaps later storytellers included this among the vampire's powers to explain the inconsistency of Mina Harker's behavior.